"It is/has been 40 years since I left college"
Which one of 'is' and 'has been' should be preferred in above sentence and why?
"It's been" a.k.a. "It has been" would be preferred because it's past tense.
"It's" can be used present tense or past tense.
e.g. "It is good" = "It's good"
e.g. "It has been" = "It's been seven hours and fifteen days..." ("Nothing Compares to You" song)
Traditional grammar books will say that the "it has been" version is 'correct'. However I would like to add some nuance to the discussion.
The meaning of the Present Perfect is to use the past to give information about the present time. For example "I have eaten" can mean, "I am not hungry (because I have eaten)". The Present Perfect is the Present Tense (I have, she has etc) with the Perfect Aspect ('before').
So by saying "It has been 40 years" I am communicating something about now that is reinforced by this fact about the 'past'. For instance, "I don't know NOW how school works because it has been 40 years since I left college." Or "Don't try and teach me the basics because it has been 40 years since I left college."
So using the Present Perfect is valid, and any traditional text book would say the same thing.
The question that is perhaps more interesting is if "It is 40 years" is possible. So what is the MEANING of the Present Simple? Well, to communicate something that is true for Past-Present-Future... My name is Adam, I work here (these are things that FEEL fixed/permanently true (even the Present Simple for Future Fixed Plans reflects this... they were set in motion in the past and 'feel' unchangeable to us [eg the plane departure time is at 7pm]) But I digress...
The choice here with the Present Simple could mean I have this now. For example "I have a college degree", I use the Present Simple even though I received it in the past because I want you to focus on the Present Permanent Fact. I could also have said "I have had a college degree for 40 years." They are both fine and they empasise different things!
HOWEVER... the fact that you went to college 40 years ago WILL change! So here using the Present Simple feels a bit strange (because the 40 years 'fact' will change in the future... to 41 years ago...) but in some circumstances it could be valid.
For example, (although this is a little 'forced' perhaps..."It is 40 years since I left college and now I am allowed to eat for free in the college restaurant!"
(For more information on the Meaning of Tenses [not the Uses of the Tenses] why not have a look at the excellent The English Verb by Michael Lewis or I even did some interactive blog posts here: Mastery of the Tenses Part 1